During a DUI investigation, South Carolina law enforcement may ask a driver to submit to a breath test. Under the state’s implied consent law, refusing to take the test will automatically result in a six-month license suspension or even longer for people who have had more than one such offense. Therefore, many people opt to take the breath tests, which unfortunately have a track record of returning unreliable results.

 The Numbers

The National Motorists Association reports that breath tests have as high as a 50 percent margin of error. The legal blood alcohol concentration limit is set at .08 for most drivers. However, based on the association’s findings, someone with a .04 BAC could still blow a .08 and get arrested for drunk driving despite having done no wrong.

 Why the Inaccuracy?

Breath tests are very susceptible to environmental factors, as a report from the State University of New York demonstrates. For example, people who work around paints or certain chemicals may be more likely to register a positive reading for alcohol even when none is present. The study also found the following:

• A breath test can interpret blood or alcohol in the mouth as alcohol.
• There are certain bread products that will register as alcohol.
• Both the temperature of the person taking the test and the air temperature can have an effect on the reading.

The National Highway Traffic Safety Administration found that people with certain conditions may also wrongly fail a breath test. People who are diabetic, for example, have higher levels of acetone in their bodies. A breath test can interpret that substance as alcohol, just as it wrongly interprets substances that are similar to the molecular structure of the ethanol found in alcohol.

Law Enforcement Mistakes

Another reason that breath tests have proven to be unreliable is that law enforcement may make mistakes while using them. For example, a machine should be recalibrated after every use. Failing to do so can result in an inaccurate reading. What’s more, the machines should be tested regularly for defects and programming errors. Lastly, an officer has a specific procedure to follow when administering the test. Failing to abide by the correct process will affect the ability of a breath test machine to register the right reading.

As the National Motorists Association points out, breath tests should not be used as per se evidence in a case regarding drunk driving. Blood tests have a much higher accuracy rate and give a better indication of how much – or how little – alcohol is in someone’s system.